How to change the grid's length values?
You have 3 options essentially. 

Two of which are cosmetic, and involve changing the grid size - but will not affect actual scale or unit measurements of your file for 3D printing or other CAM methods.


By default, 1 blender unit (and 1 grid unit) equals 1 meter. If you want to 1 blender unit to equal 1mm, you can either:
Option 1 - scale the grid via the Properties Panel (N) --> Display --> Grid Floor Scale.

Option 2 - In the Properties Window --> Scene settings --> Units, 1st set to Metric, then set the scale to 0.001.

However, if you export or save your file, 1 blender unit will still equal 1mm for most external programs. 

If you want to print your model, your safest bet will be to scale your model and regard your model's dimensions as if they are in millimeters. For instance, 155 blender units = 15.5cm.

Of course you can scale your model AND change the grid settings to make the grid nicer, no contradiction there.